(1) Field of the Invention
The present invention relates to a reference carrier generator device for use in a demodulator system which outputs quadrature demodulated signals by synchronous detection and demodulation of a quadrature modulated wave.
A demodulator system outputs quadrature demodulated signals by synchronous detection and demodulation of a quadrature modulated wave when the modulated wave is received. Generally, the demodulator system is provided with a reference carrier generator device which outputs a reference carrier synchronous with a carrier of the modulated wave.
There are several existing demodulator systems having various configurations. It is desired that a reference carrier generator device be configured to output a reference carrier that quickly responds to a change of the carrier of the quadrature modulated wave. It is desired that the reference carrier generator device provide a broad capability to pull the reference carrier into synchronism while avoiding a false stabilized point of synchronism.
(2) Description of the Related Art
FIG. 6 shows a conventional analog demodulator system.
As shown in FIG. 6, in the conventional analog demodulator system, a synchronous detection unit 51, a roll-off filter 52, a roll-off filter 53, an analog-to-digital (A/D) converter 54, an analog-to-digital (A/D) converter 55, and a transversal equalizer 56 are provided. Further, a voltage-controlled oscillator 57, a voltage-controlled clock generator 58, a reference carrier generator device 59, a control unit 63, and a low-pass filter 64 are provided.
In the conventional analog demodulator system of FIG. 6, the voltage-controlled oscillator 57 outputs a reference carrier to the synchronous detection unit 51. The voltage-controlled clock generator 58 outputs a sampling clock to both the A/D converter 54 and the A/D converter 55.
A quadrature modulated wave is produced based on, for example, quadrature amplitude modulation (QAM). In the conventional analog demodulator system of FIG. 6, an intermediate-frequency quadrature modulated wave (IN) is input to the synchronous detection unit 51 when the modulated wave is received.
In the synchronous detection unit 51, synchronous detection of an I-channel signal and a Q-channel signal from the quadrature modulated wave IN is performed based on the reference carrier from the voltage-controlled oscillator 57. The I-channel signal and the Q-channel signal, output by the synchronous detection unit 51, are passed through the roll-off filter 52 and the roll-off filter 53, and input to the A/D converter 54 and the A/D converter 55, respectively.
The I-channel signal and the Q-channel signal are converted into digital signals by the A/D converter 54 and the A/D converter 55 based on the sampling clock from the voltage-controlled clock generator 58, and the digital signals are input to the equalizer 56. The equalizer 56 outputs an I-channel demodulated signal (Iout) and a Q-channel demodulated signal (Qout). The demodulated signals Iout and Qout are further supplied to a next circuit (not shown) connected to an output of the conventional analog demodulator system.
In the reference carrier generator device 59, as shown in FIG. 6, a phase error detection unit 60, a loop filter 61, and a sweep unit 62 are provided. However, a modification may be made such that the reference carrier generator device 59 further includes the voltage-controlled oscillator 57. See Japanese Laid-Open Patent Application No.1-78058(Japanese Patent No. 2,553,103) for a background technology of the reference carrier generator device of the present invention.
In the reference carrier generator device 59 of FIG. 6, the phase error detection unit 60 detects a phase error between a reference carrier and the carrier of the quadrature modulated wave IN based on the demodulated signals Iout and Qout from the equalizer 56. Suppose that xe2x80x9cPe xe2x80x9d indicates a reference carrier phase error signal, xe2x80x9cId xe2x80x9d indicates an I-channel identifying signal, xe2x80x9cQd xe2x80x9d indicates a Q-channel identifying signal, xe2x80x9cIe xe2x80x9d indicates an I-channel phase error signal, and xe2x80x9cQe xe2x80x9d indicates a Q-channel phase error signal. The phase error detection unit 60 outputs the phase error signal Pe indicating the phase error between the reference carrier and the carrier of the quadrature modulated wave IN, which is represented by the formula:
Pe=Idxc3x97Qexe2x88x92Iexc3x97Qdxe2x80x83xe2x80x83(1)
See Japanese Laid-Open Patent Application No. 6-54014 for a background technology of the reference carrier generator device of the present invention.
In the reference carrier generator device 59 of FIG. 6, the phase error signal Pe from the phase error detection unit 60 is passed through the loop filter 61 and converted into a control voltage by the loop filter 61. The sweep unit 62 supplies the control voltage, output by the loop filter 61, to the voltage-controlled oscillator 57. The voltage-controlled oscillator 57 outputs the reference carrier to the synchronous detection unit 51 based on the control voltage from the sweep unit 62, the reference carrier being synchronous with the carrier of the quadrature modulated wave IN.
The sweep unit 62 changes the control voltage output to the voltage-controlled oscillator 57, in order to provide a broad capability to pull the reference carrier into synchronism. An error detection unit (not shown) is provided on the output side of the conventional analog demodulator system. The error detection unit detects whether an out-of-sync error of the reference carrier is increasing. When the out-of-sync error of the reference carrier is detected as increasing, it is determined that the reference carrier is going out of synchronism. The error detection unit in such a case outputs an out-of-sync alarm signal to the sweep unit 62, and the sweep unit 62 changes the control voltage upon receiving the out-of-sync alarm signal. When the out-of-sync error of the reference carrier is detected as decreasing, it is determined that the reference carrier is being pulled into synchronism. The error detection unit in such a case outputs no alarm signal to the sweep unit 62, and the sweep unit 62 stops changing the control voltage.
The control unit 63 detects a clock component based on the demodulated signals Iout and Qout from the equalizer 56, and supplies a control voltage to the voltage-controlled clock generator 58 through the low-pass filter 64. The voltage-controlled clock generator 58 outputs the sampling clock to both the A/D converter 54 and the A/D converter 55 in accordance with the control voltage from the control unit 63.
FIG. 7 shows a conventional digital demodulator system.
As shown in FIG. 7, in the conventional digital demodulator system, a quasi synchronous detection unit 71, a roll-off filter 72, a roll-off filter 73, an analog-to-digital (A/D) converter 74, an analog-to-digital (A/D) converter 75, an equalizer 76, and a phase rotation unit 77 are provided. Further, a voltage-controlled phase signal generator 78, a reference carrier generator device 79, a control unit 83, a low-pass filter 84, a voltage-controlled clock generator 85, and a local oscillator 86 are provided.
In the conventional digital demodulator system of FIG. 7, an intermediate-frequency quadrature modulated wave (IN) is input to the quasi synchronous detection unit 71 when the modulated wave is received. The local oscillator 86 outputs a fixed-frequency signal to the quasi synchronous detection unit 71. The fixed-frequency signal from the local oscillator 86 is preset such that a fixed frequency of the signal from the local oscillator 86 matches with a frequency of the carrier of the quadrature modulated wave IN.
In the quasi synchronous detection unit 71, synchronous detection of an I-channel signal and a Q-channel signal from the quadrature modulated wave IN is performed based on the fixed-frequency signal from the local oscillator 86. In this example, there is always a phase difference (and a frequency difference) between a reference carrier and the carrier of the quadrature modulated wave IN.
In the conventional digital demodulator system of FIG. 7, the phase rotation unit 77 is provided so as to eliminate a phase error between the reference carrier and the carrier of the quadrature modulated wave IN. The voltage-controlled phase signal generator 78 outputs a phase signal to the phase rotation unit 77, the phase signal indicating a controlled phase of the reference carrier. In the phase rotation unit 77, phase rotation of the I-channel I-channel signal and the Q-channel signal, both from the equalizer 76, are controlled in accordance with the phase signal from the voltage-controlled phase signal generator 78. The phase rotation unit 77 outputs an I-channel demodulated signal (Iout) and a Q-channel demodulated signal (Qout) such that the demodulated signals Iout and Qout match with the controlled phase of the reference carrier.
Similar to the example of FIG. 6, in the conventional digital demodulator system of FIG. 7, the voltage-controlled clock generator 85 outputs a sampling clock to both the A/D converter 74 and the A/D converter 75. The I-channel signal and the Q-channel signal from the quasi synchronous detection unit 71 are converted into digital signals by the A/D converter 74 and the A/D converter 75 based on the sampling clock from the voltage-controlled clock generator 85, and the digital signals are input to the equalizer 76.
The control unit 83 detects a clock component based on the demodulated signals Iout and Qout from the phase rotation unit 77, and supplies a control voltage to the voltage-controlled clock generator 85 through the low-pass filter 84. The voltage-controlled clock generator 85 outputs the sampling clock to both the A/D converter 74 and the A/D converter 75 in accordance with the control voltage from the control unit 83.
In the reference carrier generator device 79, as shown in FIG. 7, a phase error detection unit 80, a loop filter 81, and a sweep unit 82 are provided. The voltage-controlled phase signal generator 78 is equivalent to the voltage-controlled oscillator 57 in the example of FIG. 6. The voltage-controlled phase signal generator 78 outputs the phase signal to the phase rotation unit 77 based on a control voltage output from the reference carrier generator device 79.
Similar to the example of FIG. 6, in the reference carrier generator device 79 of FIG. 7, the phase error detection unit 80 detects a phase error Pe between the reference carrier and the carrier of the quadrature modulated wave IN based on the demodulated signals lout and Qout from the phase rotation unit 77. The phase error signal Pe from the phase error detection unit 80 is passed through the loop filter 81 and converted into the control voltage by the loop filter 81. The sweep unit 82 supplies the control voltage, output by the loop filter 81, to the voltage-controlled phase signal generator 78. The voltage-controlled phase signal generator 78 outputs the phase signal of the reference carrier to the phase rotation unit 77, the reference carrier being synchronous with the carrier of the quadrature modulated wave IN. In the phase rotation unit 77, the phase error between the reference carrier and the carrier of the quadrature modulated wave IN is eliminated by rotating the phase of the reference carrier.
FIG. 8 shows another conventional digital demodulator system.
As shown in FIG. 8, in the conventional digital demodulator system, a quasi synchronous detection unit 91, a roll-off filter 92, a roll-off filter 93, an analog-to-digital (A/D) converter 94, an equalizer 96, and a phase rotation unit 97 are provided. Further, a voltage-controlled clock generator 95, a voltage-controlled phase signal generator 98, a reference carrier generator device 99, a control unit 103, and a low-pass filter 104 are provided.
In the conventional digital demodulator system of FIG. 8, an intermediate-frequency quadrature modulated wave (IN) is input to the A/D converter 94 when the modulated wave is received, and the quadrature modulated wave IN is converted into a digital signal by the A/D converter 94. The digital signal from the A/D converter 94 is input to the quasi synchronous detection unit 91.
In the quasi synchronous detection unit 91, synchronous detection of an I-channel signal and a Q-channel signal from the digital signal (derived from the quadrature modulated wave IN) is performed. The phase rotation unit 97 is provided so as to eliminate a phase error between a reference carrier and the carrier of the quadrature modulated wave IN. Similar to the example of FIG. 7, the voltage-controlled phase signal generator 98 outputs a phase signal to the phase rotation unit 97, the phase signal indicating a controlled phase of the reference carrier. In the phase rotation unit 97, phase rotation of the I-channel signal and the Q-channel signal, both from the equalizer 96, are controlled in accordance with the phase signal from the voltage-controlled phase signal generator 98. The phase rotation unit 97 outputs an I-channel demodulated signal (Iout) and a Q-channel demodulated signal (Qout) such that the demodulated signals Iout and Qout match with the controlled phase of the reference carrier.
In the conventional digital demodulator system of FIG. 8, the voltage-controlled clock generator 95 outputs a sampling clock to the A/D converter 94. The A/D converter 94 outputs the digital signal to the quasi synchronous detection unit 91 based on the sampling clock from the voltage-controlled clock generator 95.
Similar to the example of FIG. 7, as shown in FIG. 8, the reference carrier generator device 99 is essentially the same as the reference carrier generator device 79 of FIG. 7. The control unit 103 and the low-pass filter 104, both shown in FIG. 8, are essentially the same as the control unit 83 and the low-pass filter 84, both shown in FIG. 7. A duplicate description thereof will be omitted.
FIG. 9 shows a configuration of a sweep unit in the demodulator systems of FIG. 6 through FIG. 8. The sweep unit of FIG. 9 corresponds to one of the sweep units 62, 82 and 102 in the reference carrier generator devices of FIG. 6, FIG. 7 and FIG. 8.
In FIG. 9, a loop filter 111, a sweep unit 112, and a voltage-controlled oscillator (VCO) 116 are provided. In the sweep unit 112, as shown in FIG. 9, an adder 113, a sweep signal generator 114, and a switch (SW) 115 are provided.
In the sweep unit of FIG. 9, a phase error signal is passed through the loop filter 111, and converted into a control voltage by the loop filter 111. The sweep unit 112 supplies the control voltage, output by the loop filter 111, to the voltage-controlled oscillator 116. The voltage-controlled oscillator 116 outputs a reference carrier to the above-mentioned synchronous detection unit based on the control voltage from the sweep unit 112, the reference carrier being synchronous with the carrier of the quadrature modulated wave IN.
When no out-of-sync alarm signal (ALM) is input to the switch 115, it is determined that the reference carrier is pulled into synchronism, and the switch 115 is set in an OFF state. The sweep unit 112 stops changing the control voltage output to the oscillator (VCO) 116. The sweep unit 112 supplies the control voltage to the oscillator (VCO) 116 based on the phase error signal from the loop filter 111. A phase of an output signal of the oscillator (VCO) 116 is controlled such that the reference carrier from the oscillator (VCO) 116 quickly responds to a phase error between the reference carrier and the carrier of the quadrature modulated wave.
When an out-of-sync alarm signal (ALM) is input to the switch 115, it is determined that the reference carrier is going out of synchronism under a certain operating condition, and the switch 115 is set in an ON state. A sweep signal, which has, for example, a triangular waveform as shown in FIG. 9, is supplied from the sweep signal generator 114 to the adder 113 through the switch 115. The sweep signal of the triangular waveform from the sweep signal generator 114 is added to the phase error signal from the loop filter 111 by the adder 113. Thus, the sweep unit 112 changes the control voltage output to the oscillator (VCO) 116 upon receiving the out-of-sync alarm signal (ALM), in order to provide a broad capability to pull the reference carrier into synchronism. The oscillator (VCO) 116 outputs the reference carrier based on the changed control voltage, the reference carrier being synchronous with the carrier of the quadrature modulated wave IN. When the reference carrier from the oscillator (VCO) 116 is pulled into synchronism, no out-of-sync alarm signal (ALM) is output to the switch 115 and the switch 115 is again set in the OFF state.
Similar to the example of FIG. 9, in the reference carrier generator devices 59, 79 and 99 in the conventional demodulator systems of FIGS. 6, 7 and 8, a sweep signal having a triangular waveform is added to a phase error signal when an out-of-sync alarm signal (ALM) is received, and the control voltage output to the oscillator (VCO) 116 is changed upon receiving the out-of-sync alarm signal (ALM), in order to provide a broad capability to pull the reference carrier into synchronism.
However, in the conventional demodulator systems, there may be a case in which the reference carrier from the voltage-controlled oscillator is set at a false stabilized point of synchronism. In such a case, the sweep unit 112 (the sweep unit 62, 82 or 102) supplies a control voltage to the voltage-controlled oscillator 116 (the oscillator 57 or the phase signal generator 78 or 98) based on a phase error signal from the loop filter 111, the phase error signal having a waveform opposite to the triangular waveform of the sweep signal from the sweep signal generator 114. The reference carrier from the voltage-controlled oscillator is continuously fixed at a false stabilized point of synchronism. Once the reference carrier is set at a false stabilized point, it is difficult for the reference carrier generator devices of the conventional demodulator systems to pull the reference carrier out of the false stabilized point into a correct stabilized point of synchronism.
Generally, in the conventional demodulator systems, the loop filter 111 is configured by using a digital filter, and the digital filter quickly responds to a change of the carrier of the quadrature modulated wave. In a case in which the loop filter 111 is configured by using a digital filter, it is further difficult for the reference carrier generator devices of the conventional demodulator systems to pull the reference carrier out of the false stabilized point into a correct stabilized point of synchronism.
An object of the present invention is to provide an improved reference carrier generator device in which the above-mentioned problems are eliminated.
Another object of the present invention is to provide a reference carrier generator device which has a simple configuration and easily and quickly pulls a reference carrier out of a false stabilized point into a correct stabilized point of synchronism even after the reference carrier is set at the false stabilized point.
The above-mentioned objects of the present invention are achieved by a reference carrier generator device which includes: a phase error detection unit which detects a phase error between a reference carrier and a carrier of a quadrature modulated wave and outputs a phase error signal indicating the phase error; a loop filter which passes the phase error signal from the phase error detection unit through the loop filter and converts the phase error signal into a control voltage; a voltage-controlled oscillator which outputs a reference carrier synchronous with the carrier of the quadrature modulated wave based on the control voltage; and a sweep unit which changes the control voltage output to the oscillator when pulling the reference carrier into synchronism in response to an out-of-sync alarm signal, wherein the sweep unit outputs a staircase sweep signal having a selected level, the staircase sweep signal being added to the phase error signal such that the control voltage output to the oscillator is changed by the staircase sweep signal.
The above-mentioned objects of the present invention are achieved by a reference carrier generator device which includes: a phase error detection unit which detects a phase error between a reference carrier and a carrier of a quadrature modulated wave and outputs a phase error signal indicating the phase error; a loop filter which passes the phase error signal from the phase error detection unit through the loop filter and converts the phase error signal into a control voltage; a voltage-controlled oscillator which outputs a reference carrier synchronous with the carrier of the quadrature modulated wave based on the control voltage; and a sweep unit which changes the control voltage output to the oscillator when pulling the reference carrier into synchronism in response to an out-of-sync alarm signal, wherein the sweep unit outputs a sweep signal having a selected level which discontinuously varies from one of a plurality of discrete values to another, the sweep signal being added to the phase error signal such that the control voltage output to the oscillator is changed by the sweep signal.
The reference carrier generator device of the present invention includes the sweep unit which changes the control voltage output to the voltage-controlled oscillator when pulling the reference carrier into synchronism in response to an out-of-sync alarm signal. According to one aspect of the present invention, the sweep unit outputs a staircase sweep signal having a selected level, the sweep signal being added to the phase error signal such that the control voltage output to the oscillator is changed by the staircase sweep signal. According to another aspect of the present invention, the sweep unit outputs a sweep signal having a selected level which discontinuously varies from one of a plurality of discrete values to another, the sweep signal being added to the phase error signal such that the control voltage output to the oscillator is changed by the sweep signal. Therefore, the reference carrier generator device of the present invention is effective in quickly pulling the reference carrier into synchronism while avoiding a false stabilized point of synchronism. Even if the reference carrier is set at a false stabilized point of synchronism, the reference carrier generator device of the present invention is effective in easily and quickly pulling the reference carrier out of the false stabilized point into a correct stabilized point of synchronism.